The invention relates to a semiconductor component with alternating Schottky and pn junctions and low-doped drift zones of a semiconductor material, which are arranged between the Schottky and pn junctions, as well as a method for producing a component.
In addition to pn diodes, Schottky diodes are also used as components for low operating voltages. Schottky diodes are distinguished through low on-state voltages and low switching losses. In order to reduce the field peaks occurring at the edge of the Schottky junction, so-called guard rings are frequently provided at the edge of the component. These guard rings reduce the field peaks occurring at the edge of the Schottky junction and advantageously contribute to the increase in the breakdown voltage of the component.
However, the series resistances in the component as well as the off-state leakage currents increase with an increase in the breakdown voltage of the diode, particularly with increased temperatures. A broader use of the Schottky diodes, which are in reality very simple technologically, is thus made more difficult.
The strong dependence of the off-state leakage current on the voltage, owing to the voltage-induced deformation of the energetic barrier of the Schottky junction, presents a particular problem. The Schottky barrier is reduced at the barrier by an applied off-stage voltage and therewith connected electrical field, so that the off-state leakage current increases strongly with the off-state voltage and can have very high values, even prior to the actual breakdown. In addition, the off-state currents show an exponential increase along with the temperature, owing to the underlying thermal emission mechanism, which results in an unfavorable rejection characteristic.
Schottky diodes made from different semiconductor materials are known. In the EP 380 340 A2, a Schottky diode made of SiC is described. In the article by L. Wang et al., xe2x80x9cHigh Barrier Height GaN Schottky diodes: Pt/GaN and Pd/GaNxe2x80x9d in Appl. Phys. Lett. 68(9), Feb. 26, 1996 1267-1269, Schottky diodes made of GaN are disclosed. The German Patent 42 10 402 A1 discloses Schottky diodes made of diamond lattice.
In literature, various approaches are described for improving the rejection characteristic, e.g. in B. M. Wilamowski, xe2x80x9cSchottky Diodes with High Breakdown Voltages,xe2x80x9d Solid-State Electronics, vol. 26(5), p. 491-493, 1983, and in B. J. Baliga, xe2x80x9cThe Pinch Rectifier: A Low-Forward-Drop High-Speed Power Diode,xe2x80x9d IEEE Electron Device Letters, EDL-5(6), 1984. It is assumed therein that a screening reduces the electrical field intensity at the Schottky junction. The component described therein is a so-called xe2x80x9cMerged-pn/Schottky (MPS) Rectifier,xe2x80x9d which has inside the guard ring arrangement alternating Schottky contacts and highly doped pn junctions with n-drift zones of a semiconductor material arranged in-between. To be sure, the rejection characteristic of these components is improved, but there are several disadvantages.
In addition to the loss of active surface for the Schottky junctions, in particular the injection of minority charge carriers from the highly doped semiconductor area is a disadvantage during the forward operation of the pn contacts. With a forward polarity of the MPS component, the current initially flows only over the Schottky regions. With further increasing forward voltage, the pn junctions also enter the passage range, wherein minority charge carriers are injected into the drift zone. In contrast to the components having only guard ring arrangements, this minority charge carrier injection cannot be neglected since the charge carrier injection can even result in the formation of electron-hole plasma, in the same way as for pure pn diodes. On the one hand, the passage properties are slightly improved while, on the other hand, the switching losses increase sharply.
Literature describes a series of measures for improving the switching behavior and rejection characteristic of the MPS components. The U.S. Pat. No. 5,262,669 A discloses arranging the pn junctions for MPS components in etched grooves, as well as adapting the geometry of the pn junctions to the barrier height of the Schottky junction or the space charge zone that forms at the Schottky junction. The technologies and arrangements used, however, are technologically very involved. For that reason, the MPS components are practically unimportant, despite their generally simple production technology and advantageous characteristics of unipolar diodes.
It is the object of the invention to provide a MPS component for which the switching losses are improved without the use of involved technological steps.
This object generally is solved by a semiconductor component with outer contacts as anode and cathode and a guard ring and comprising alternately arranged Schottky junctions and pn junctions arranged near the surface of the semiconductor component, as well as low-doped drift zones of a semiconductor material that are arranged between the Schottky junctions and the pn junctions; and wherein the energy difference (Egapxe2x88x92"PHgr"barrier) between the band gap (Egap) in the electronic excitation range of the drift zone semiconductor material and the energetic height ("PHgr"barrier) of the Schottky barrier is at least 0.8 eV in the non-voltage state of the semiconductor component and the band gap (Egap) is higher than 1.5 eV. Modified and advantageous embodiments follow from the description.
The invention concerns an arrangement with a junction between Schottky metal and drift zone of a semiconductor, as well as a screening of pn junctions, which screening is essentially embedded in the above junction. A minimum difference between energetic height of the Schottky barrier and the energy gap of the semiconductor material must be maintained here for the selection of materials for semiconductor and Schottky metal.
It is advantageous if the simple technology of the MPS components can be used and the rejection characteristics as well as the shutdown losses are improved.
It is particularly advantageous if semiconductors with high-energy gap, particularly so-called xe2x80x98wide-band gapxe2x80x99 semiconductors, are used. It is favorable if the material for the Schottky contact is selected such that it does not fall below a minimum barrier height. The rejection characteristics are thus improved.
In one preferred embodiment, the drift zone consists of silicon carbide, while in another embodiment, the drift zone consists of gallium nitride. In a further embodiment, the drift zone consists of aluminum nitride while that of another one consists of diamond lattice.
In one preferred embodiment, a drift zone with identical type of conductivity but lower doping is arranged on a highly doped substrate material of silicon carbide. It is advantageous if a substrate material doping of higher or equal to 1018 cmxe2x88x923 and a drift zone doping of 1014 to 1017 cmxe2x88x923 is used.
One advantageous embodiment has a drift zone with a thickness of between 2 xcexcm and 50 xcexcm.
The distance between adjacent pn junctions for one preferred embodiment is between 0.5 xcexcm and 20 xcexcm.
In another preferred embodiment, the pn junctions are arranged in grooves that are etched into the inner drift zone.
The Schottky junctions of one preferred embodiment are arranged adjacent to the pn junctions in the drift zone.
The highly doped regions in the drift zone of one preferred embodiment are formed by the drift zone semiconductor material with complementary doping.
In another preferred embodiment, the highly doped regions in the drift zone are formed by different semiconductor materials.
One favorable embodiment has aluminum and/or boron inserted into the doping region for the p-doped substrate material.
Nitrogen and/or phosphor are inserted in one favorable embodiment for the n-doped substrate material.
The cathode and anode of another preferred embodiment are arranged on opposite surfaces of the semiconductor component.
One preferred embodiment provides that the cathode and anode are arranged on the same semiconductor component surface. In particular, one outer contact encloses the other outer contact.
An essentially pointed contacting occurs between an outer contact and the drift zone of one preferred embodiment.
In another preferred embodiment, a highly doped semiconductor region is positioned in front of the cathode, which region has the same type of conductivity as the drift zone of the semiconductor.
It is particularly favorable that a component according to the invention can also be designed as complementary doped component.
A method according to the invention specifies the production of a silicon carbide component according to the invention. In one preferred embodiment, a particularly favorable temperature treatment is carried out. The advantage is that the surface roughness of the component is not made worse during the temperature treatment.